Boron phosphide-based semiconductor light-emitting device

ABSTRACT

A boron phosphide-based semiconductor light-emitting device includes a substrate of silicon single crystal, a first cubic boron phosphide-based semiconductor layer that is provided on a surface of the substrate and contains twins, a light-emitting layer that is composed of a hexagonal Group III nitride semiconductor and provided on the first cubic boron phosphide-based semiconductor layer and a second cubic boron phosphide-based semiconductor layer that is provided on the light-emitting layer, contains twins and has a conduction type different from that of the first cubic boron phosphide-based semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dateof Provisional Application No. 60/553,531 filed Mar. 17, 2004 pursuantto 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a boron phosphide-based semiconductorlight-emitting device having a boron phosphide-based semiconductor layerexhibiting a wide bandgap, which device can emit high-intensity light inspite of a lattice-mismatch structure thereof.

BACKGROUND ART

Conventionally, an n-type or a p-type boron phosphide (BP)-basedsemiconductor layer has been employed for fabricating light-emittingdiodes (LEDs) and laser diodes (LDs). For example, JP-A HEI 5-283744discloses that a blue-LED is fabricated from a semiconductor structureincluding a silicon substrate, and an n-type BP layer to which silicon(Si) has been intentionally added and an aluminum gallium nitride(AlGaN) layer successively formed on the substrate. The prior art alsodiscloses that a magnesium (Mg)-doped p-type BP layer is employed as acontact layer for fabricating an LED (see paragraph [0023] in the priorart).

As mentioned above, boron phosphide exhibiting a bandgap of 2.0 eV atroom temperature is employed in combination with a Group III nitridesemiconductor, such as Al_(X)Ga_(Y)In_(1-X-Y)N (0≦X≦1, 0≦Y≦1), forfabricating a compound semiconductor light-emitting device (see, forexample, JP-A HEI 2-288388). In the aforementioned LED emitting bluelight of a wavelength corresponding to such a wide bandgap, a boronphosphide layer specifically serves as a base layer on which a Group IIInitride semiconductor layer is grown, rather than as a cladding layer ora similar layer (see paragraph [0013] in JP-A HEI 5-283744).

In the case where a boron phosphide layer serving as a base layer isformed on a crystalline substrate, such as a silicon single-crystalsubstrate, it is known that the plane orientation of a surface of anepitaxially grown boron phosphide layer is determined in accordance withthe crystal plane orientation of the surface of the substrate. Forexample, JP-A HEI 5-283744 discloses in paragraph [0025] that a (100)boron phosphide layer is grown on a (100) crystal plane of a siliconsubstrate and that a cubic AlGaInN layer is grown on the (100) crystalplane of the (100) boron phosphide layer. On the other hand, it is knownthat a (111) boron phosphide layer is grown on a (111) crystal plane ofthe silicon substrate and that a hexagonal AlGaInN layer is grown on the(111) crystal plane of the (111) boron phosphide layer.

The cubic AlGaInN, which is a promising candidate for a light-emittinglayer or a similar layer, has a crystal structure less stable than thatof a hexagonal Group III nitride semiconductor (see paragraph [0002] inJP-A HEI 5-283744). Thus, the cubic semiconductor cannot be formed in astable state as compared with a hexagonal Group III nitridesemiconductor, which is problematic.

As mentioned above, efforts have been made for growing a hexagonalAlGaInN layer having a more stable crystal structure on the (111)crystal plane of the boron phosphide layer formed on the (111) crystalplane of the silicon substrate. However, a portion in the hexagonalcrystalline layer containing no cubic crystals is formed only in alimited portion from the junction interface with the boron phosphidebase layer to the thickness less than 50 nm (see paragraph [0025] inJP-A HEI 5-283744).

In other words, even though it is intended that a hexagonal Group IIInitride semiconductor layer is formed in a sufficient thickness on aboron phosphide-based semiconductor layer having a (111) siliconsubstrate, actual formation of the semiconductor layer isproblematically difficult.

The present invention has been accomplished in view of the foregoing.Thus, an object of the present invention is to provide a boronphosphide-based semiconductor light-emitting device in which ahigh-crystallinity, hexagonal Group III semiconductor layer is formed ina sufficient thickness on a boron phosphide-based semiconductor layerprovided on a silicon substrate, leading to manifestation of highemission intensity.

DISCLOSURE OF THE INVENTION

To attain the above object the present invention provides a boronphosphide-based semiconductor light-emitting device comprising asubstrate of silicon single crystal, a first cubic boron phosphide-basedsemiconductor layer that is provided on a surface of the substrate andcontains twins, a light-emitting layer that is composed of a hexagonalGroup III nitride semiconductor and provided on the first cubic boronphosphide-based semiconductor layer and a second cubic boronphosphide-based semiconductor layer that is provided on thelight-emitting layer, contains twins and has a conduction type differentfrom that of the first cubic boron phosphide-based semiconductor layer.

In the first mentioned device, the substrate is a (111)-siliconsingle-crystal substrate having a (111) crystal plane, and the firstcubic boron phosphide-based semiconductor layer is provided on the (111)crystal plane.

In the second mentioned device, the first cubic boron phosphide-basedsemiconductor layer has a [110] direction aligned with a [110] directionof the silicon single crystal.

In the second or third mentioned device, the first cubic boronphosphide-based semiconductor layer contains (111) twins having a (111)crystal plane serving as a twinning plane in a junction area in contactwith the (111) crystal plane of the (111)-silicon single-crystalsubstrate.

In any one of the first to fourth mentioned devices, the first cubicboron phosphide-based semiconductor layer is an undoped layer to whichno impurity element has been intentionally added.

In any one of the first to fifth mentioned devices, the light-emittinglayer has a [−2110] direction aligned with a [110] direction of thefirst cubic boron phosphide-based semiconductor layer and has a (0001)crystal plane serving as a front surface.

In any one of the first to sixth mentioned devices, the light-emittinglayer has a profile of phosphorus atom concentration that graduallydecreases from a bottom thereof in a thickness direction.

In the sixth mentioned device, the second cubic boron phosphide-basedsemiconductor layer has a [110] direction aligned with the [−2110]direction of the light-emitting layer.

In any one of the sixth to eighth mentioned devices, the second cubicboron phosphide-based semiconductor layer contains (111) twins having a(111) crystal plane serving as a twinning plane in a junction area incontact with the (0001) crystal plane of the light-emitting layer.

In any one of the sixth to ninth mentioned devices, the second cubicboron phosphide-based semiconductor layer is an undoped layer to whichno impurity element has been intentionally added.

In any one of the first to tenth mentioned devices, the first and secondcubic boron phosphide-based semiconductor layers exhibit a bandgap atroom temperature of 2.8 eV or more.

In any one of the first to eleventh mentioned devices, the first andsecond cubic boron phosphide-based semiconductor layers are provided soas to serve as cladding layers.

In any one of the first to eleventh mentioned devices, the second cubicboron phosphide-based semiconductor layer is provided so as to serve asa window layer which allows passage of light emitted from thelight-emitting layer to the outside.

In any one of the first to eleventh mentioned devices, the second cubicboron phosphide-based semiconductor layer is provided so as to serve asa current-diffusion layer which allows device operation current todiffuse.

In any one of the first to eleventh mentioned devices, the second cubicboron phosphide-based semiconductor layer is provided so as to serve asa contact layer for forming an electrode.

The boron phosphide-based semiconductor light-emitting device accordingto the present invention is composed of a silicon single-crystalsubstrate, a first cubic boron phosphide-based semiconductor layer thatis provided on a surface of the silicon single-crystal substrate andcontains twins, a light-emitting layer that is composed of a hexagonalGroup III nitride semiconductor and provided on the first cubic boronphosphide-based semiconductor layer and a second cubic boronphosphide-based semiconductor layer that is provided on thelight-emitting layer and contains twins. In other words, upon growth ofthe first cubic boron phosphide-based semiconductor layer on thesilicon-single crystal substrate that is highly lattice-mismatched withthe layer, twins are incorporated into the junction area of the firstcubic boron phosphide-based semiconductor layer, whereby latticemismatch between the layer and the substrate can be mitigated. Then, ahexagonal Group III nitride semiconductor light-emitting layer isprovided on the first cubic boron phosphide-based semiconductor layerwhose lattice-mismatch has been mitigated. Therefore, the thus formedlight-emitting layer has excellent crystallinity and can possess asufficient layer thickness, thereby attaining manifestation of highemission intensity.

Upon growth of a second cubic boron phosphide-based semiconductor layeron the hexagonal Group III nitride semiconductor light-emitting layer,twins are incorporated into the junction area of the second cubic boronphosphide-based semiconductor layer, whereby lattice mismatch betweenthe light-emitting layer and the second cubic boron phosphide-basedsemiconductor layer can be mitigated, thereby reducing lattice mismatchof the second cubic boron phosphide-based semiconductor layer. Thus, alight-emitting device exhibiting excellent blocking voltagecharacteristics with few local breakdowns can be fabricated.

The above and other objects, characteristic features and advantages ofthe present invention will become apparent to those skilled in the artfrom the description to be made herein below with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of a stacked structureemployed for fabricating an LED according to the present inventionhaving a double-hetero (DH) junction structure.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments for carrying out the present invention will next bedescribed in detail.

The boron phosphide-based semiconductor employed in the presentinvention contains, as essential elements, boron (B) and phosphorus (P).Examples thereof include B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ)(0<α≦1, 0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1),B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1, 0≦β<1, 0≦γ<1,0<α+β+γ≦1, 0≦δ<1), boron monophosphide (BP), boron phosphide, galliumindium boron phosphide (B_(α)Ga_(γ)In_(1-α-γ)P (0<α≦1, 0≦γ<1)) and mixedcrystal compounds containing a plurality of Group V elements, such asboron nitride phosphide (BP_(1-δ)N_(δ) (0≦δ<1)) and boron arsenidephosphide (B_(α)P_(1-δ)As_(δ) (0≦α≦1, 0≦δ≦1))

According to the Grimm-Sommerfeld's rule (see Lecture of BasicIndustrial Chemistry 5, Inorganic Industrial Chemistry, published byAsakura Shoten, 6th edition, p. 220, Feb. 25 (1973)), a Group III-Vcompound semiconductor composed of a Group III element, such as aluminum(Al) or gallium (Ga), and a Group V element, such as phosphorus (P) orarsenic (As), may take either cubic sphalerite crystal form or hexagonalwurtzite crystal form. Conventionally, hexagonal boron phosphide isemployed as a base layer for growing a Group III nitride semiconductorlayer (see the aforementioned JP-A HEI 5-283744). However, according tothe present invention, a boron phosphide-based semiconductorlight-emitting device is fabricated from a boron phosphide semiconductorlayer of cubic sphalerite crystal form. In cubic sphaleritesemiconductor crystals, energy levels on the valence band side aredegenerated. Therefore, as compared with wurtzite semiconductorcrystals, cubic sphalerite semiconductor crystals readily provide ap-type conductor layer suitable for a cladding layer or a similar layer.

The present invention employs a silicon single-crystal substrate havinga diamond-type crystal structure that is identical to the crystalstructure of a boron phosphide-based semiconductor layer of cubicsphalerite crystal form in order to ensure formation of the boronphosphide-based semiconductor layer of cubic sphalerite crystal form. Ona silicon (Si) crystal substrate, a boron phosphide-based semiconductorlayer is formed through the halogen method, the hydride method, MOCVD(metal-organic chemical vapor deposition) or a similar method.Alternatively, molecular-beam epitaxy may also be employed. In anexemplary procedure, a boron monophosphide layer is formed through MOCVDby use of triethylborane ((C₂H₅)₃B) and phosphine (PH₃).

Even though the surface of a silicon single-crystal substrate has acrystal plane orientation other than (111) (e.g., (100) or (110)), a(111) boron phosphide-based semiconductor layer having a stacked (111)crystal plane structure can be readily formed when the source supplyratio (i.e., V/III ratio) at film formation is increased during growthof the boron phosphide-based semiconductor layer through theaforementioned vapor growth means. However, if a (111) siliconsingle-crystal substrate is employed from the beginning, a (111) boronphosphide-based semiconductor layer can be grown even at a low V/IIIratio. Therefore, in the present invention, a (111) siliconsingle-crystal substrate having a surface with a (111) crystal plane isemployed as a substrate. The (111) silicon single crystal has a firstconduction type. When a stacked structure for use in light-emittingdevices is fabricated, a boron phosphide-based semiconductor layerhaving a first conduction type is deposited on the siliconsingle-crystal substrate of the first conduction type.

Through employment of a (111) silicon single-crystal substrate having asurface with a (111) crystal plane, a (111) boron phosphide-basedsemiconductor layer can be grown even at a low V/III ratio. Thesingle-crystal substrate is advantageous for forming a p-type BP layerin an undoped state. For example, even when MOCVD is performed at aV/III ratio (PH₃/(C₂H₅)₃B supply ratio) as low as 10 to 50, a p-type(111) boron phosphide-based semiconductor layer can be readily grown at1,000° C. to 1,200° C. In addition to the growth temperature and V/IIIratio, through precise control of growth rate, a (111) boronphosphide-based semiconductor layer having a wide bandgap can be formedon a (111) silicon single-crystal substrate. The growth rate is suitablycontrolled to 2 to 30 nm/min.

During vapor-phase growth of a (111) boron phosphide-based semiconductorlayer (e.g., a boron monophosphide layer) on a (111) siliconsingle-crystal substrate, when the growth rate at an initial growthstage is increased, twins are effectively generated in a region in thevicinity of the junction with the substrate. In the case where thelattice mismatch degree with respect to the silicon single crystal islarger, twins are readily generated in a junction region in contact withthe substrate without greatly increasing the growth rate. For example,in a junction region between a silicon single crystal (latticeconstant=5.4309 Å) and boron phosphide (lattice constant=4.5383 Å)(i.e., lattice mismatch degree of about 16.4%), twins can be generatedat a growth rate of 20 nm/min and an areal density of about 5×10¹¹ cm⁻².The areal density of twins decreases in the thickness direction from thebottom of the boron phosphide-based semiconductor layer. The arealdensity of twins can be determined through, for example, counting thenumber of twins within a predetermined region in a cross-section TEMimage captured under a transmission electron microscope.

The twins generated in the junction region can mitigate lattice mismatchbetween the silicon single-crystal substrate and the boronphosphide-based semiconductor layer, thereby providing a boronphosphide-based semiconductor layer having excellent crystallinity. Inthe cubic sphalerite boron phosphide-based semiconductor layer, thetwins preferably have a (111) crystal plane serving as a twining plane.Among twins, (111) twins are particularly effective for mitigatinglattice mismatch between the (111) silicon single-crystal substrate andthe boron phosphide-based semiconductor layer. The presence or absenceof (111) twins in the boron phosphide semiconductor layer can beobserved on the basis of anomalous diffraction spots in an electron-beamdiffraction image.

The (111) boron phosphide-based semiconductor layer whose crystallinityhas been enhanced through generation of (111) twins and which exhibits awide bandgap may be employed in a compound semiconductor light-emittingdevice as a barrier layer, such as a cladding layer. Particularly, aboron phosphide-based semiconductor layer exhibiting a bandgap at roomtemperature of 2.8 eV or more, desirably 3.5 eV or more is preferablyemployed. For example, the cladding layer is preferably formed from alow-resistance boron phosphide-based semiconductor layer having, at roomtemperature, a carrier concentration of 1×10¹⁹ cm⁻³ or more and aresistivity of 5×10⁻² Ω·cm or less. The p-type boron phosphide-basedsemiconductor layer for forming the cladding layer preferably has athickness of 50 to 5,000 nm.

A cladding layer or a similar layer that is composed of a (111) boronphosphide-based semiconductor and formed on a (111) siliconsingle-crystal substrate is effective for forming thereon a hexagonalwurtzite Group III nitride semiconductor layer. On a boronphosphide-based semiconductor layer having a surface with a (111)crystal plane, a light-emitting layer having a crystal plane orientationof (0001), e.g., gallium indium nitride (Ga_(X)In_(1-X)N (0≦X ≦1)) orgallium nitride phosphide (GaN_(1-Y)P_(Y) (0≦Y≦1)) can be grown.Ga_(X)In_(1-X)N (0≦X≦1) or a similar semiconductor in which the [−2110]direction (a-axis) of bottom plane lattice is aligned with the [110]direction of a (111) boron phosphide-based semiconductor layer issuitable for forming a light-emitting layer. When the Ga_(X)In_(1-X)Nhas a multi-phase structure including a plurality of phases havingdifferent indium compositions (=1−X), a boron phosphide-basedsemiconductor light-emitting device exhibiting higher emission intensityis effectively produced.

When forming a light-emitting layer of a Group III nitridesemiconductor, with the phosphorus atom concentration decreased in adirection from the first cubic boron phosphide-based semiconductor layer(on the substrate) to the second cubic boron phosphide-basedsemiconductor layer (on the light-emitting layer), the formedlight-emitting layer exhibits excellent adhesion to the first cubicboron phosphide-based semiconductor layer and emits high-intensitylight. For example, after completion of vapor-phase growth of the firstboron phosphide-based semiconductor layer, a phosphorus (P) source gasemployed for the growth is gradually discharged to the outside of thegrowth system, while a nitrogen source gas for growing a Group IIInitride semiconductor layer serving as a light-emitting layer isgradually supplied into the growth system, whereby a light-emittinglayer having a graded phosphorus atom concentration can be formed. Inthis case, the period of time for discharging the phosphorus source tothe outside of the growth system, is kept prolonged, the phosphorus atomconcentration decrease profile in the light-emitting layer becomesgentle. The phosphorus atom concentration at the bottom (on the firstboron phosphide-based semiconductor layer side) of the light-emittinglayer is preferably 5×10¹⁸ cm⁻³ to 2×10²⁰ cm⁻³ from the viewpoint ofadhesion between the two layers. The phosphorus atom concentration atthe top (on the second boron phosphide-based semiconductor layer side)of the light-emitting layer is preferably controlled to 5×10¹⁹ cm⁻³ orless from the viewpoint of emission intensity. The phosphorus atomconcentration profile in the light-emitting layer may be determinedthrough secondary ion mass spectrometric analysis (SIMS) or a similarmethod.

The (0001) crystal plane of the wurtzite Group III nitride semiconductorlayer having the aforementioned orientation conditions is effective forforming thereon a (111) cubic boron phosphide-based semiconductor layer.When a (0001) Group III nitride semiconductor layer (e.g., a GaN layerhaving a (0001) surface) is employed, a (111) boron phosphide-basedsemiconductor layer can be readily grown in a “double positioning”manner (see P. HIRSCH et al., “ELECTRON MICROSCOPY OF THIN CRYSTAL,”Krieger Pub. Com. (1977, U.S.A.), p. 306). A (111) boron phosphide-basedsemiconductor layer in which the [110] direction is aligned with the[−2110] direction exhibits less lattice strain and is effectivelyemployed as a cladding layer or a window layer which allows passage ofvisible light (blue light, green light, etc.) to the outside of alight-emitting device.

Upon growth of a (111) boron phosphide-based semiconductor layer on asurface of a (0001) Group III nitride semiconductor layer, when (111)twins are incorporated into a junction region, a (111) boronphosphide-based semiconductor layer having remarkably excellentcrystallinity can be formed. In order to form twins in the junctionregion, the growth rate of the (111) boron phosphide-based semiconductorlayer is modified. Specifically, in contrast to the case in which a(111) boron phosphide-based semiconductor layer is grown on the (111)silicon single-crystal substrate, the growth rate at an initial growthstage is reduced. For example, the growth rate is preferably 2 to 10nm/min. When the growth rate is gradually elevated as the increase inlayer thickness (e.g., to 20 to 30 nm/min) to thereby grow a boronphosphide-based semiconductor layer in a short period of time, loss ofhighly volatile element, such as phosphorus (P), is prevented, and aboron phosphide-based semiconductor layer having a conduction type andcarrier concentration of interest can be obtained.

The (111) boron phosphide-based semiconductor layer exhibiting a widebandgap provided on the (0001) Group III nitride semiconductor layer maybe employed as a cladding layer, a window layer or a contact layer. Whenthe bandgap is in excess of about 5 eV, the energy level gap between thesemiconductor layer and the light-emitting layer excessively increases,and production of a boron phosphide-based semiconductor light-emittingdevice exhibiting low forward voltage or low threshold voltage isimpaired, although it is advantageous for transmitting emitted light.The bandgap can be determined on the basis of wavelength dispersibilityof refractive index and extinction coefficient. Regardless of the typeof the layer (i.e., cladding layer, window layer or contact layer), anundoped boron phosphide-based semiconductor layer to which no impurityelement is intentionally added is effective for preventing unwantedmodification of other layers caused by diffusion of a doped impurityelement.

The boron phosphide-based semiconductor light-emitting device of thepresent invention is fabricated by providing an ohmic electrode of afirst polarity on a surface of a cladding layer, a window layer or acontact layer, each composed of the boron phosphide-based semiconductorlayer containing the aforementioned twins, and by providing an ohmicelectrode of a second polarity on the backside of a siliconsingle-crystal substrate or a similar layer. On an n-type boronphosphide-based semiconductor layer, an n-type ohmic electrode may beformed from gold-germanium (Au—Ge) alloy or a similar material, whereason a p-type boron phosphide-based semiconductor layer, a p-type ohmicelectrode may be formed from gold-zinc (Au—Zn) alloy, gold-beryllium(Au—Be) alloy or nickel (Ni) alloy. When a wide-area LED having a sidelength of 500 μm or longer is fabricated, it is effective that aplurality of small, circular (e.g., diameter: 20 to 50 μm) ohmicelectrodes are provided over a wide area of the boron phosphide-basedsemiconductor layer surface, and these electrodes are electricallyconnected to one another. Through employment of the electrodeconfiguration, device operation current can be diffused over a widesurface of the layer, which is advantageous for fabricating LEDsexhibiting high emission intensity or having wide emission area.

EXAMPLES

The present invention will next be described in detail, with referenceto fabrication of a boron phosphide-based LED including a (111) boronphosphide (BP) layer formed on a (111) silicon single-crystal substrateand a (0001) gallium indium nitride light-emitting layer formed on the(111) BP layer.

FIG. 1 schematically shows a cross section of a stacked structureemployed for fabricating an LED according to the present inventionhaving a double-hetero (DH) junction structure. In FIG. 1, a stackedstructure 11 is provided for fabricating an LED chip 10.

The stacked structure 11 was formed by sequentially stacking on aphosphorus-doped n-type (111) silicon (Si) single-crystal substrate 101an undoped n-type (111) boron phosphide lower cladding layer 102, amulti-quantum well structure light-emitting layer 103 includingrepeatedly (3 times) stacked an n-type (0001) gallium indium nitride(Ga_(0.90)In_(0.10)N) well layer and a (0001) gallium nitride barrierlayer, and an undoped p-type (111) boron phosphide upper cladding layer104.

The bottom layer in the light-emitting layer 103 which was in contactwith the lower cladding layer 102 was a well layer, and on the welllayer, a barrier layer, a well layer, a barrier layer, a well layer anda barrier layer were sequentially stacked. The uppermost barrier layerwas in contact with the upper cladding layer 104.

The undoped n-type (111) boron phosphide layer (lower cladding layer102) and the undoped p-type (111) boron phosphide layer (upper claddinglayer 104) were formed through a normal pressure (near atmosphericpressure) metal-organic vapor phase epitaxy (MOVPE) means by use oftriethylborane ((C₂H₅)₃B) as a boron source and phosphine (PH₃) as aphosphorus source. The n-type (111) boron phosphide layer (lowercladding layer 102) and the p-type (111) boron phosphide layer (uppercladding layer 104) were formed at 925° C. and 1,025° C., respectively.The light-emitting layer 103 was formed through a trimethylgallium((CH₃)₃Ga)/NH₃/H₂) reaction atmospheric pressure MOCVD means at 800° C.The aforementioned gallium indium nitride layer forming the well layershad a multi-phase structure including a plurality of phases havingdifferent indium compositions. The average indium composition was foundto be 0.10 (=10%). Each well layer had a thickness of 5 nm, and eachbarrier layer had a thickness of 10 nm.

In an initial stage of growing a (111) boron phosphide layer serving asa lower cladding layer 102 on the surface of the (111) siliconsingle-crystal substrate 101, the growth rate was controlled to 25nm/min. Until the layer thickness reached 50 nm, growth was performed atthe same rate. Subsequently, the growth rate was reduced to 20 nm/minand the growth was continued until the total layer thickness reached 600nm. On the other hand, in an initial stage of growing the upper claddinglayer 104 on the light-emitting layer 103 composed of a (0001) Group IIInitride semiconductor, the growth rate was controlled to 10 nm/min andthen the growth rate was increased to 20 nm/min, thereby growing thep-type upper cladding layer 104 having a total thickness of 200 nm. Aportion of the layer grown at a low growth rate of 10 nm/min had athickness of 25 nm.

The undoped n-type (111) boron phosphide layer serving as the lowercladding layer 102 was found to have a carrier (electron) concentrationof 6×10¹⁹ cm⁻³ and a resistivity at room temperature of 8×10⁻³ Ω·cm. Theundoped p-type (111) boron phosphide layer serving as the upper claddinglayer 104 was found to have a carrier (hole) concentration of 2×10¹⁹cm⁻³ and a resistivity at room temperature of 5×10⁻² Ω·cm.

The bandgap at room temperature was determined on the basis of photonenergy dependency of a doubled value (=2n·k) of the product (=n·k) ofrefractive index (n) and extinction coefficient (k) As a results, then-type (111) boron phosphide layer serving as the lower cladding layer102 was found to have a bandgap of 3.1 eV, and the p-type (111) boronphosphide layer serving as the upper cladding layer 104 was found tohave a bandgap of 4.2 eV. Thus, the upper cladding layer 104 composed ofp-type boron phosphide was regarded as a candidate for a p-type claddinglayer also serving as a window layer for transmitting light emitted fromthe light-emitting layer 103.

A selective area electron-beam diffraction (SAD) pattern obtained froman inside portion of the n-type (111) boron phosphide layercorresponding to a junction area between the (111) siliconsingle-crystal substrate 101 and the lower cladding layer 102 (i.e., aportion from the junction interface with the substrate 101 to athickness of 50 nm of the n-type boron phosphide layer) containedanomalous diffraction spots attributed to (111) twin crystals. Theseanomalous spots were regularly patterned between {111} diffraction spotsand had a spacing ⅓ that of {111} diffraction spots, indicating thatthese twins were identified as (111) twins.

Another selective area electron-beam diffraction (SAD) pattern obtainedfrom an inside portion of the p-type (111) boron phosphide layercorresponding to a junction area between the (0001) Group III nitridesemiconductor light-emitting layer 103 and the upper cladding layer 104(i.e., a portion from the junction interface with the multi-quantum wellstructure light-emitting layer 103 to a thickness of 25 nm of the p-typeboron phosphide layer) also contained anomalous diffraction spotsattributed to twin crystals. Thus, (111) twins were observed in bothjunction regions.

The lattice image of each of the aforementioned junction regions wascaptured through a conventional cross-section TEM technique, and thenumber of (111) twins was counted from the image. The areal density of(111) twins in a region in the vicinity of the junction interfacebetween the (111) silicon single-crystal substrate 101 and the n-typelower cladding layer 102 was found to be about 6×10¹¹ cm⁻². The arealdensity of (111) twins gradually decreased in the thickness direction,and was found to be 7×10⁸ cm⁻² in a region in the vicinity of thesurface of the n-type lower cladding layer 102.

The areal density of (111) twins in the p-type (111) boron phosphidelayer corresponding to the junction region between the (0001) Group IIInitride semiconductor light-emitting layer 103 and the upper claddinglayer 104 was found to be about 2×10¹⁰ cm⁻². The areal density of (111)twins drastically decreased and was found to be about 5×10⁷ cm⁻² in aregion in the vicinity of the surface of the upper cladding layer.

The orientation feature of each of epitaxially grown layers 102 to 104was investigated through observation under a conventional transmissionelectron microscope (TEM). Specifically, TED patterns with respect to anincident electron beam parallel to the [110] direction of the (111)silicon single-crystal substrate 101 were captured. From the n-type(111) boron phosphide layer serving as the lower cladding layer 102, areverse lattice pattern with respect to the (110) crystal plane wasobtained, indicating that the [110] direction of the n-type (111) boronphosphide layer was aligned with the [110] direction of the siliconsingle-crystal substrate 101. The TED patterns also indicated that thep-type (111) boron phosphide layer (upper cladding layer 104) was grownsuch that the [110] direction was aligned with the [−2110] direction ofthe hexagonal Group III nitride semiconductor light-emitting layer 103.

After completion of the growth of the lower cladding layer 102, supplyof phosphine (PH₃) gas employed for growing the lower cladding layer 102to the growth system was gradually decreased from a flow rate of 430cc/min to 0 cc/min over five seconds, rather than immediately stoppingthe supply. In order to investigate the effect of the above operation onthe phosphorus atom concentration profile in the light-emitting layer103, the phosphorus atom concentration profile in the light-emittinglayer 103 in the thickness direction was analyzed through conventionalSIMS. As a result, the well layer closest to the lower cladding layer102 had a mean phosphorus atom concentration of about 9×10¹⁹ cm⁻³. Thewell layer present in the mid portion of the light-emitting layer 103had a mean phosphorus atom concentration of about 2×10¹⁹ cm⁻³.

The well layer closest to the upper cladding layer 104 had a meanphosphorus atom concentration of about 6×10¹⁸ cm⁻³, indicating that thephosphorus atom concentration decreased in the thickness direction ofthe light-emitting layer 103.

On the entire surface of the p-type boron phosphide layer serving as theupper cladding layer 104 and also as a window layer through whichemitted light is extracted, a gold-germanium (Au—Ge) alloy film, anickel (Ni) film and a gold (Au) film were sequentially depositedthrough conventional vacuum vapor deposition. Subsequently, the metalfilms were selectively patterned through a known photolithographictechnique such that the aforementioned tri-layer electrode having abottom surface formed of the Au—Ge alloy film remained exclusively inthe center portion of the upper cladding layer 104 where a p-type ohmicelectrode 105 also serving as a pad electrode for wire bonding was to beprovided. Other than the area where the p-type ohmic electrode 105 wasprovided, the metal films were removed through etching so as to exposethe surface of the upper cladding layer 104. After removal ofphotoresist material, the cladding layer was selectively patterned againso as to provide lattice-pattern grooves for cutting the structure intochips. Thereafter, the thus formed lattice-pattern of the upper claddinglayer 104 was exclusively removed through plasma dry etching employing achlorine-containing halogen mixture gas to thereby form grooves forcutting the structure into chips.

On the entire backside of the silicon single-crystal substrate 101, agold (Au) film was deposited through a conventional vapor depositiontechnique, and an n-type ohmic electrode 106 was formed from the goldfilm. The structure was cleaved along the aforementioned slip-likegrooves having a line width of 50 μm and being provided parallel to the[110] direction normal to the (111) surface of the siliconsingle-crystal substrate 101, thereby producing square (350 μm×350 μm)LED chips 10.

Emission characteristics of the LED chips 10 were evaluated when forwarddevice operation current (20 mA) was caused to flow between the p-typeohmic electrode 105 and the n-type ohmic electrode 106. The LED chips 10were found to emit blue light having a wavelength of 440 nm. The halfwidth of an emission peak observed in an emission spectrum was found tobe 220 meV. The luminance of the light emitted from each chip beforeresin-molding, as determined through a typical integrating sphere, was10 mcd. The forward voltage (Vf) at a forward current of 20 mA was foundto be as low as 3.1 V, whereas the reverse voltage at a reverse currentof 10 μA was found to be as high as 9.5 V. Virtually no local breakdownswere observed.

INDUSTRIAL APPLICABILITY

As described hereinabove, according to the present invention, alight-emitting layer composed of a (0001) Group III nitridesemiconductor is caused to be joined with a (111) boron phosphide-basedsemiconductor layer containing (111) twins which mitigatelattice-mismatch with a silicon substrate or a similar material, in thecase where a boron phosphide-based semiconductor light-emitting deviceis fabricated from a boron phosphide-based semiconductor layer grown ona highly lattice-mismatch silicon single-crystal substrate. Therefore, alight-emitting layer having high crystallinity can be produced, and aboron phosphide-based semiconductor light-emitting device fabricatedfrom the light-emitting layer can emit high-intensity light.

In addition, the upper cladding layer which is joined with alight-emitting layer composed of a (0001) Group III nitridesemiconductor and which also serves as a window layer is formed from alow-lattice-strain (111) boron phosphide semiconductor layer containing(111) twins which layer is oriented such that the [110] direction isaligned with the [−2110] direction of the light-emitting layer.Therefore, boron phosphide-based semiconductor LEDs and similar devicesexhibiting excellent blocking voltage characteristics with few localbreakdowns can be provided.

1. A boron phosphide-based semiconductor light-emitting devicecomprising: a substrate of silicon single crystal; a first cubic boronphosphide-based semiconductor layer that is provided on a surface of thesubstrate and contains twins; a light-emitting layer that is composed ofa hexagonal Group III nitride semiconductor and provided on the firstcubic boron phosphide-based semiconductor layer, said light-emittinglayer having a multi-quantum well structure comprising a plurality ofwell layers; and a second cubic boron phosphide-based semiconductorlayer that is provided on the light-emitting layer, contains twins andhas a conduction type different from that of the first cubic boronphosphide-based semiconductor layer, wherein the light-emitting layerhas a profile of phosphorus atom concentration that monotonicallydecreases from a bottom to a top thereof in a thickness direction suchthat the phosphorus atom concentration among the plurality of welllayers is highest for the well layer closest to the substrate and islowest for the well layer farthest from the substrate, and a phosphorusatom concentration at a bottom of the light-emitting layer is 5×10¹⁸cm⁻³to 2×10²⁰cm⁻³.
 2. A boron phosphide-based semiconductor light-emittingdevice according to claim 1, wherein the substrate is a (111)-siliconsingle-crystal substrate having a (111) crystal plane, and the firstcubic boron phosphide-based semiconductor layer is provided on the (111)crystal plane.
 3. A boron phosphide-based semiconductor light-emittingdevice according to claim 2, wherein the first cubic boronphosphide-based semiconductor layer has a [110] direction aligned with a[110] direction of the silicon single crystal.
 4. A boronphosphide-based semiconductor light-emitting device according to claim2, wherein the first cubic boron phosphide-based semiconductor layercontains (111) twins having a (111) crystal plane serving as a twinningplane in a junction area in contact with the (111) crystal plane of the(111)-silicon single-crystal substrate.
 5. A boron phosphide-basedsemiconductor light-emitting device according to claim 1, wherein, thefirst cubic boron phosphide-based semiconductor layer is an undopedlayer to which no impurity element has been intentionally added.
 6. Aboron phosphide-based semiconductor light-emitting device according toclaim 1, wherein the light-emitting layer has a [−2110] directionaligned with a [110] direction of the first cubic boron phosphide-basedsemiconductor layer and has a (0001) crystal plane serving as a frontsurface.
 7. A boron phosphide-based semiconductor light-emitting deviceaccording to claim 6, wherein the second cubic boron phosphide-basedsemiconductor layer has a [110] direction aligned with the [−2110]direction of the light-emitting layer.
 8. A boron phosphide-basedsemiconductor light-emitting device according to claim 6, wherein thesecond cubic boron phosphide-based semiconductor layer contains (111)twins having a (111) crystal plane serving as a twinning plane in ajunction area in contact with the (0001) crystal plane of thelight-emitting layer.
 9. A boron phosphide-based semiconductorlight-emitting device according to claim 6, wherein the second cubicboron phosphide-based semiconductor layer is an undoped layer to whichno impurity element has been intentionally added.
 10. A boronphosphide-based semiconductor light-emitting device according to claim1, wherein the first and second cubic boron phosphide-basedsemiconductor layers exhibit a bandgap at room temperature of 2.8 eV ormore.
 11. A boron phosphide-based semiconductor light-emitting deviceaccording to claim 1, wherein the first and second cubic boronphosphide-based semiconductor layers are provided so as to serve ascladding layers.
 12. A boron phosphide-based semiconductorlight-emitting device according to claim 1, wherein the second cubicboron phosphide-based semiconductor layer is provided so as to serve asa window layer which allows passage of light emitted from thelight-emitting layer to the outside.
 13. A boron phosphide-basedsemiconductor light-emitting device according to claim 1, wherein thesecond cubic boron phosphide-based semiconductor layer is provided so asto serve as a current-diffusion layer which allows device operationcurrent to diffuse.
 14. A boron phosphide-based semiconductorlight-emitting device according to claim 1, wherein the second cubicboron phosphide-based semiconductor layer is provided so as to serve asa contact layer for forming an electrode.